libvirtconnection: refactor: open a new connection in special function

This patch splits opening of a new connection to qemu:///system into a
new function, distinct of further decorations. get() is a cumbersome
function that could use some trimming.

Change-Id: I33f83d20d1906dcba7b3652abaeddcba14bec6c4
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28348
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-by: mooli tayer <mtayer@redhat.com>